How postal codes in Japan are structured?

A postal code identifies a district. This site follows this structure: top level is for provinces, second level for cities and third one for districts.

Notice that streets are not used in addresses in Japan. Most of streets don't have a name. Even if they have one, it is not used in the address. It is like a nickname or trade name used to make easier to identify the place. No more than that.

So how a house or building is specified?
Each district is divided into blocks. Then into sub-blocks. Then each property inside the sub-block is uniquely numbered (they are NOT sequential numbers). So you have something like "1-2-3" describing block, sub-block and property respectively. Another variation is 1 chome-2-3. "Chome" can be omitted.

Bigger cities have another layer between cities and districts: wards. To keep it simple, the postal code system considers each of those wards an individual city. For example, Sendai city is broken into 5: "Sendai-shi, Aoba-ku", "Sendai-shi, Izumi-ku", "Sendai-shi, Miyagino-ku", "Sendai-shi, Taihaku-ku" and "Sendai-shi, Wakabayashi-ku".

This does not apply to Tokyo because, for postal purposes, Tokyo is a province rather than a city.

Some zip codes are uniquely assigned to specific building. Others are for a specific floor of a specific building.

These are the basics . As you can expect from any human construct, there is a lot of exceptions and ambiguity. But at least you have the basics here that will help you to understand the information in this directory.